Mineral separation procedures

Procedures for preparation of a rock sample for chemical and/or isotopic analysis and for a mineral (zircon, monazite, baddeleyite, titanite, xenotime etc) mount for SIMS analysis are outlined below.

Good Laboratory Practice
Preparation and analysis of a rock sample for geochronology is very lengthy and time-consuming (as will be evident from the details given here)– this effort can be rendered futile if even a single zircon grain from elsewhere is accidentally introduced as a contaminant into the sample. Therefore, particular emphasis must be taken in adopting clean procedures for the processing of samples for geochronology so that any possibility of laboratory cross-contamination is minimized.

Walls, surfaces and floors should be thoroughly washed down and then wiped with clean water prior to commencement of use of a rock crushing and milling laboratory for geochronology processing. During use of the laboratories, meticulous care should be taken to maintain laboratory cleanliness. Entry should be restricted to staff trained in or directly involved in sample processing. All laboratory surfaces and laboratory equipment should be carefully cleaned prior to processing of each sample. All water used in the laboratory should be filtered to remove >10 µm size particles.

Equipment Requirements

Hydraulic press: for fracturing of larger rock samples in preparation for rock crusher

Large rock saw: for surface trimming of rock samples of weathered surfaces and removal of veins

Conventional rock crusher: should not have inner and inaccessible surfaces which are hard to clean and on which dust can settle; all components in contact with samples must be capable of being dismantled for routine thorough cleaning between samples. A rock crusher preferably should have a simple, vertical rock/chip path to the chip collection tray, which must be easily cleanable.

Rock mill: disk mill preferred, must be capable of being readily dismantled and reassembled for thorough cleaning between samples. An alternative to conventional crushing and milling (Steps 3 and 4) is Selfrag (selective fragmentation system; see Selfrag webpage).

Elutriator including stands: specialized glass funnel apparatus that will require fabrication by a glass blower. At least 3 glass funnels are required for optimal sample throughput.

Sample Pre-treatment
Each rock sample should be initially trimmed of all weathered surfaces, alteration and secondary veins using a rock saw. Following thorough washing in water to remove loose rock dust and drying, the trimmed pieces are then ready to be broken up to cm-sized fragments using a hydraulic press, in preparation for the crusher.

Pre-cleaning of rock crushing/defrag and milling equipment
Rock crushing and milling equipment should be dismantled and thoroughly cleaned to remove all rock dust and loose material from previously processed samples. Crusher jaws should be scrubbed with a wire brush to remove rock dust. The crusher’s internal parts and chip collection tray should be washed and cleaned thoroughly using compressed air and examined carefully for cleanliness.

Milling equipment should be completely dismantled and the disks and rings thoroughly washed and completely dried (using compressed air if possible) on a clean sheet of A3 paper.

All lab-ware should be thoroughly washed in detergents and distilled water and dried before use. Prepare new sieve cloths for each sample and dispose of these after each use. Place sample chips and powders on new A3 sheets of paper or in pre-cleaned stainless steel trays during processing steps; store in plastic bags.

Sample Crushing/Defragmentation and Milling
Samples may be passed through the jaw crusher to produce fragments with sizes suitable for milling. To minimize cross-contamination, any fine powder produced by the rock crusher should be discarded and only the larger chips used for the milling step. The fragments are then ground, in small (less than 120 g) batches, in a tungsten carbide or low-chromium steel ring-mill using the minimum grinding time necessary for the resultant powder to pass through a disposable nylon sieve cloth with mesh size 150 to 400 µm (depending on the nature and grain size of the sample). A milling step may not be required if a Defrag can produce disaggregated minerals that will pass through the mesh size 150 to 400 µm disposable nylon sieve cloth and the process is guaranteed to be free of cross-contamination.

Between 0.5 and 3 kg of milled powder should be typically required for the isolation of zircon, although up to 5 kg of coarsely crushed material should routinely be prepared from each sample, to facilitate acquisition of complementary major- and trace-element geochemical analyses.

All prepared rock powder should be stored in clearly labeled, new double-layered clip-sealable plastic bags.

Elutriation
In order to remove low-density fine material, the sieved powder should then be ‘elutriated’ in 300 g batches with filtered water, under controlled flow conditions (400 to 2400 dl/min), using a 2000 ml glass funnel apparatus. The water flow conditions within this apparatus may be calibrated, using a sample having known heavy-mineral concentrations, to ensure that fine, low-density material is thoroughly removed and that no heavy-minerals are lost in the fine fraction. The funnels are fitted with plastic taps near their base to enable the water flow to be turned on and off whilst the sample is in the funnel. The water supply hoses to each funnel have joining connectors so that the supply hoses can be easily disconnected. After the elutriation process has been completed and no more light minerals are passing through the upper funnel outlet, the lower tap can be closed. The heavier minerals will settle to the bottom of the funnel and can be drained from the funnel when the tap is opened.

Sodium polytungstate
After drying of the residue in an oven, less than 150 g quantities of the heavy fraction are thoroughly mixed by shaking with concentrated sodium polytungstate solution (Na6[H2W12O40].H2O, density 2.9 g/cm3) in 1000 ml glass separating funnels. The funnels are periodically agitated to facilitate density separation and may be left overnight to ensure complete density separation. The heavy-mineral concentrate is drained from each funnel onto a filter paper, washed thoroughly with distilled water and acetone in a Büchner vacuum funnel, and oven-dried overnight at 90ºC.

Notes on the use of sodium polytungstate
When using sodium polytungstate, use only glass, plastics or 100% stainless steel laboratory equipment and only use warm distilled, demineralized or deionized water. Tap water may contain calcium ions, which will react with sodium polytungstate to form insoluble calcium metatungstate. Wash all lab equipment with warm distilled or demineralized or deionized water before using with sodium polytungstate. In order to expedite dissolution, always add sodium polytungstate to water, not vice versa.

Do not allow the solution to come into contact with reducing agents, which will cause a blue or yellowish discoloration. An oxidation agent like hydrogen peroxide (H2O2) can convert it back to the original color. Avoid organic materials, which can cause a brownish discoloration. To avoid the yellow color after recycling the sodium polytungstate solution, wash the resin thoroughly several times and letting it sit in saturated NaCl to reconstitute the Na-resin, then rinse the salt with one bed volume of water.

The polytungstate solution can be recovered by washing samples and labware repeatedly with distilled, demineralized or deionized water. To filter off the solids, including impurities and remaining crystals, from the liquid, use a funnel with Millipore 0.45 micrometer filter papers. Coffee filters may also be used.

All minerals and solids should be removed before evaporation. Re-concentrate the sodium polytungstate solution in an oven at 100°C to140°C, depending on the concentration. If the temperature is too high, the solution may splatter, whereas when the temperature is too low, it will take longer to evaporate. Never evaporate the solution to a solid, as this will be very difficult to re-dissolve and that may be very difficult to remove mechanically from glass containers. To re-dissolve the concrete-like precipitate, add distilled water to the solid and exercise patience, as it will take quite some time to re-dissolve.

To decrease or increase the density, dilute the solution with distilled water or add solid or evaporate the solution to increase the density.

Retarded crystallization from over-saturated solution is a common phenomenon with this type of compound. The crystals may be re-dissolved but the solution recovered by this means may be unstable and starts crystallizing within a few days to a couple of weeks. By heating the solution and slowly evaporating it, it is possible to obtain a clear, high-density solution.

Store the solution only in plastic or stainless steel containers, not in glass vials. The solution is chemically aggressive and will dissolve calcium ions from the glass, resulting in formation of insoluble calcium metatungstate. Do not to store the solution for long periods, as crystals will precipitate.

For a density of 2.94g/ml, use 840g sodium polytungstate and 160g H2O to give 1000g solution with a volume of 340 ml.

Magnetic Separation
Highly magnetic minerals are removed from the dried heavy-mineral concentrate using a hand magnet. The heavy-mineral concentrate may then be passed through a Franz isodynamic magnetic separator. For the first pass, a longitudinal tilt of 20º, a transverse tilt of 10º, and a magnet current of 0.2 – 0.8 A are employed. The non-magnetic fraction from the first pass is reprocessed using higher tilt settings and current values that vary on an individual sample basis. All fractions from each sample are retained in storage.

Methylene Iodide (MI)
This step must be conducted in an exhausting fume-hood and using protective clothing and protective reagent gloves. MI is carcinogenic and extreme care must be taken to avoid breathing the fumes or coming into contact with MI.

The least magnetic fraction is then treated with methylene iodide using a sensitive, miniaturized ‘double-tube’ method. This involves the use of a 16 mm diameter test tube into which a 1.5 mm diameter constriction has been placed, so that an open, bell-shaped chamber is present at its base. The constriction is small enough to hold up the heavy-liquid in the upper chamber by its meniscus, when the top of the tube is stoppered. This is placed into a larger test tube that contains methylene iodide. The heavy-mineral concentrate is carefully poured into the inner tube, its contents are gently stirred, and the inner tube may then be gently rotated and tilted. Heavier minerals fall past the constriction to settle in the outer test tube. When separation is complete the inner tube is stoppered, slowly lifted out, and the contents washed onto a filter paper using acetone. The purified zircon concentrate in the outer test tube is collected on a second filter.

Hand Picking
At least 100 representative zircons or monazites are hand picked in a glass petri-dish from the heavy-mineral separate under a binocular microscope. These should be carefully positioned on double-sided tape on a large thick glass slide.

Mounting in Epoxy
The hand-picked grains are then mounted in 2.54 cm diameter epoxy disks and sectioned approximately in half, and the mount surface then polished to expose the grain interiors. Each mount will typically contain hand-picked zircon populations from two samples and will also include several fragments of a standard mineral (zircon or monazite).

For mounting samples the best epoxys to use are Epofix (Struers, Copenhagen) or EPIREZ EPIMOUNT two-part epoxys, or a similar hard, transparent two-part epoxy. Also required for mount preparation are 1″ moulds, 2″ double-sided tape, and some large rectangular glass thin section blanks (granite size, 7.5 cm x 5 cm x 3 mm) or some 5 mm thick glass sheet.

Minerals should be arranged on double-sided tape (Selleys is the preferred brand as the layer of glue is thinner than most tapes), which is stuck down on a sheet of glass, and then the mould place around them. Specimens or different populations should be well separated, but centrally located. All grains should be within a central 1.7 mm diameter region to prevent grains being too close to the surface of the surrounding sample holder. Standard chips should be located close to the unknowns. Do not press the grains into the double-sided tape as this can increase the number of grains plucked from the surface during polishing. The exception to this statement is the standard chip; if these are large chips, push them down gently to ensure a large surface exposure. Ensure there is no hair or dirt on the double-sided tape surface. Don’t mark the tape with a pen as the ink will be incorporated into the epoxy. Use the mould to make a slight impression in the tape surface so that the grains can be located. The mould should be very lightly greased on the inner surface prior to pressing onto the double-sided tape. A very light smearing that is difficult to detect is about the correct amount.

Care must be taken to guarantee success with the epoxy. Weigh out the mix on a balance, as this is superior to making a mix by volume. There must be no air bubbles in the mount. These can be eliminated by very gently stirring (folding) the epoxy mix for five minutes with a spatula until the two components are completely mixed (e.g. there is no apparent streaking due to variation of refractive index). Then, ultrasone the EPIREZ mixture for a few minutes before casting. Use a fine needle to break bubbles and to ensure bubbles adjacent to the mounted grains are removed. Pipette the epoxy very carefully using a disposable pipette, to prevent the grains moving in the mount (an easy way to cross contaminate samples; if you have the time or lack experience then mount each population separately). If you do decide to put more than one population on a mount, then pipette the epoxy onto the centre of the mount as this allows the epoxy to flow outwards to the rim and this reduces the possibility of cross-contamination. Curing should be done for >24 hrs at 20 to 60°C, with the mould covered with Al foil to prevent dust entering. After it has set, carefully remove the mold and leave the disk attached to the double-sided tape.

The outcome should be a hard 1-inch diameter disk (the required size is slightly less than 1 inch diameter; 23 mm diameter is the best size, and the disk can be ground down to this size from a 1-inch mould size). The disk must be less than 5.5 mm thick and flat on both sides with both front and back faces parallel, but thick enough to have structural rigidity whilst being polished. They are usually made about 6 mm thick and then the back of the mount is ground down until the mount is 5 mm thick. A reasonable polish on the back also makes it easy to locate the standard and unknowns and should help remove any small bubbles on the surface. There should be no wedging of the disk, as this will cause problems when the disk is placed in the sample holder or instrument.

Mount Polishing
Grinding and polishing of the sample is commenced initially using grinding paper, followed by a polishing wheel. The suction attachment on a thin-section grinding wheel is preferred to hold the large-thin section slide. In this way it is easy to obtain a parallel back surface. Once the back surface is flat and the disk is the correct thickness it can be removed from the slide. Usually this can be done by peeling up the tape and tugging on it. If the disk doesn’t come off easily then gently heat the slide and the glue should soften. Use of force should not be required. The disk should initially have a flat surface, so that grinding can be reduced to a minimum. Use a 5 or 10 µm lap or 1200 grit to grind down until you are about one third of the way through the minerals. This may be monitored by comparing the observed size of the grains in both reflected and transmitted light. Then clean up the surface with 5 µm grit. A final polish using a grit size of 1 µm diamond is recommended.

Some useful hints with grinding and polishing: a) let the powder do the cutting, so don’t push down too hard, or use excessive weights. b) Use a figure-8 pattern for manual work. c) Use lots of lubricant and don’t let the polishing cloth dry out, as this will prevent friction, which is the major cause of grain plucking. d) Maintain high standards of cleanliness; wash your hands between each stage and ultrasone the sample, as cross contamination at this stage is common. The final polish must be perfect at 1 µm or less, and the surface dead flat.

It is possible to recover specimens, if necessary, by dissolving the epoxy in di-chloro-methane. Care should be taken with this chemical as it is highly carcinogenic.

Mount Photography
The mount is then photographed at x150 magnification in reflected and transmitted light to reveal internal structures within the zircons. The presence of zonation microstructures, such as rims and cores, may be further investigated using a scanning electron microscope and back-scattered electron (BSE) imaging or using cathodoluminescence (CL) imaging. These two imaging techniques essentially provide the same information.

Each mount should then be ultrasonically cleaned in Petroleum Ether, detergent (Decom) and propanol, rinsed in millipore water and then dried in an oven at 40°C. It is then ready to be evaporatively coated with high-purity Au prior to analysis.